An appeal to emotion is an argument tactic that is very similar to an appeal to popularity which attempts to circumvent rational thought in the hopes of “supporting” a conclusion with an emotional response in the place of real evidence. Many times, this fallacy is committed in concert with other fallacies as well, e.g.ad hominem

The premise of this argument is faulty; Hitler was actually a devout Catholic. Even if the premise were granted, however, the attempted link between atheism and Nazism relies on an enthymeme (missing premise).

Counterarguments

Whether or not something is appealing has no bearing on whether it is true. Not everything that is real is pleasant, and not everything an evil person does is evil.

For example:

"Hitler was a vegetarian. Do you want to be like Hitler?"

"Hitler wore a mustache. Do you want to be like Hitler?"

"Hitler believed in the theory of gravity. Do you want to be like Hitler?"

The fact that Hitler was a vegetarian and sported a mustache does not mean that those things are immoral. And the fact that he believed in gravity does not make it untrue.

Similarly:

"This X-ray shows that you have a tumor on your liver. You don't want to have liver cancer, do you? Therefore, the X-ray is wrong."

This is an inverted form of wishful thinking: wanting something to be true doesn't make it true, and wanting something to be false doesn't make it false.